#include <iostream>
#include <string>
using std::cout;
using std::endl;
using std::string;


void test(){
    string s1("hello");
    cout<<s1<<endl;
    s1+="world";
    cout<<s1<<endl;
    s1+='!';
    cout<<s1<<endl;
}

int main()
{
    

    test ();
    return 0;
}

